This invention relates generally to weighing systems and particularly relates to an improved weighing apparatus for the high-speed weighing of a cyclically dispensed flowable material and method therefor.
An important area in the field of high speed packaging involves the cyclical dispensing of a supply of particulate materials into sequential batches of uniform weight which are then automatically deposited into a package. Typically a scale is used for weighing the particulate material provided to the scale by means of a hopper, a conveyor, or other means of material transport, and upon reaching a predetermined weight, the flowable material is then deposited in the package. A check weighing approach is generally employed in these package filling machines in which each batch of material is rapidly weighed and those batches which do not fall within some acceptable weight range which includes the ideal weight of the batches are either rejected or modified so as to fall within these packaging weight limits.
Various sources of error are present in these package filling machines resulting in the apparent weight which is being sensed by the weighing means not necessarily being the true weight of the product in the scale receptacle. Random vibrations in the weighing receptacle and elements to which it is connected caused by external forces may affect the apparent weight of the substance on the scale. In addition, the weighing means may not be in its true position due to its response time which is the time it takes for the weighing means to complete its vertical displacement due to a weight being added thereto. This vertical displacement is amplified due to the vertical impact force created by the product in suspension falling on the product already located in the receptacle of the weighing means. Various inputs may also be provided by the means or manner in which the weighing container gate is opened and closed during the discharge of the substance being weighed. The true weight of the product can be check weighed only after the weighing means reaches substantial equilibrium or before equilibrium of the check weighing means has been reached by compensating for erroneous inputs resulting therefrom. To wait for equilibrium may result in a substantial lengthening of the weighing cycle while compensating for a non-equilibrium situation generally requires sophisticated sensors and associated circuitry.
One approach to the check weighing of a charge of product is disclosed in U.S. Pat. No. 4,100,984 to Klopfenstein et al wherein is described a system having a single receptacle, such as a weigh bucket, in which the material to be weighed is provided from a feeding means. While in the receptacle, the initial charge is check weighed and if the weight sensed is less than a predetermined value, additional material is provided to the receptacle until it is within a preferred range of prescribed weights. Provision is made for several check weighing operations after the additional charge of product has been provided to the receptacle with the flow of product stopped when the preferred package weight is realized. Provision is made for either a pulsed feed or a continuous feed of the additional charges of product provided to the receptacle after a check weighing operation until minimum package weight is reached.
U.S. Pat. No. 2,930,569 to Peterson discloses means for controlling the flow of material from a supply hopper. This control means includes a horizontal disc or plate rotatably mounted beneath a stationary plow blade which is positioned in such an attitude that rotation of the disc urges the material thereon against the plow blades and thereby directs the material over the edge of the disc to the inlet, or mouth, of the hopper. The discharge portion of the hopper includes several gates the positions of which are controlled by air cylinder driven piston rods. Similarly, a scale bucket positioned beneath the hopper outlet is provided with dump gates each actuated by means of a piston rod-air cylinder combination. This apparatus is primarily directed toward providing a uniform bulk and dribble flow to the weighing means and is not concerned with eliminating some of the aforementioned inaccuracies inherent in the weighing process itself.
U.S. Pat. No. 2,914,310 to Bahrs addresses the problem of scale equilibrium by disclosing an electrical damping system in which an electrical signal proportional to the displacement of the weighing platform is generated, amplified and applied to an electromechanical transducer which applies a restoring force to the weighing platform whereby the platform is maintained at or near its equilibrium position and platform movement is damped.
U.S. Pat. No. 3,656,337 to McDonald discloses a device for the periodic and regular calibration of a weighing scale beam used to weigh objects being transported on a continuously moving conveyor belt. Automatic zero adjustment of the weighing scale beam is accomplished by loading a reference weight at the end of the scale beam during a test interval, recording the reference weight on a strip chart recorder, comparing the recorded weight with a predetermined recorded output, and generating a correction signal in response to the comparison and providing the correction signal to a pulse controller in adjusting the load signal produced by the reference weight.
While the aforementioned references address some of the more conventional limitations of prior art high-speed packaging and weighing systems, they do not deal with the problem of variations from cycle to cycle in the weight of the scale elements or material container itself. Weight readings may vary substantially as a result of variations in system temperature and the presence of residue in the weigh container after discharge. These weight variations may become particularly critical where the substance being packaged is of a somewhat glutinous consistency. By compensating for the material thus retained, or the tare weight of the container as it is frequently designated, each subsequently filled increment may be accurately weighed and dumped so that over an extended period of time weighing errors are effectively minimized. Since the material stuck or hung-up in the container is weighed with its companion material when first filled, when it subsequently drops through the container, no error is created. U.S. Pat. No. 3,261,415 to Hood et al discloses a device for feeding solid materials in a continuous process at a given weight for a fixed time interval including a re-zeroing circuit which includes coupled amplifiers and associated circuitry and a control switch. With the switch closed immediately after a dump and before a fill, if the amplifier has an output signal due to material stuck in the container, it is fed back through the re-zero circuit to cancel the signal at the input of the amplifier thus indicating virtually zero output of the amplifier and therefore no material in the container before filling with a new increment.
U.S. Pat. No. 3,474,874 to Pettis also discloses a system for checking tare weight on a cycle-to-cycle basis to avoid batch weight errors due to variations in tare weight. The '874 patent involves the use of a reversible motor which compensates for an incorrect tare weight by operating in such a direction as to compress the effective length of a scale in a predetermined increment so that the scale's zero point coincides with the tare weight. The reversible motor is energized by the outputs of two switching circuits which represent the total weight of the bucket assembly and of the fixed batch weight. If this combined weight exceeds a predetermined value, the reversible motor is driven to change the effective length of the scale so as to compensate for the scale's tare weight.
Ideally, a high-speed, cyclic package filling machine would produce packages of uniform weight all of which are either at the minimum package weight desired, or in excess thereof by such a small amount that material loss is minimized. Such a system would incorporate all of the above-discussed characteristics thus eliminating, or at least minimizing, the aforementioned packaging problems.